Polyaxial bone screw

Abstract

The present invention generally provides a polyaxial fixation device having a shank with a spherical head formed on a proximal end thereof, and a receiver member having an axial passage formed therein that is adapted to polyaxially seat the spherical head of the shank. The polyaxial bone screw further includes an engagement member that is adapted to provide sufficient friction between the spherical head and the receiver member to enable the shank to be maintained in a desired angular orientation before locking the spherical head within the receiver member.

The invention claimed is: 
     
       1. A polyaxial bone anchor assembly, comprising:
 a shank having a spherical head formed on a proximal end thereof; 
 a receiver member adapted to receive a spinal rod and having a distal opening though which the shank extends and a distal seat in which the head of the shank is polyaxially seated; 
 a compression cap disposed within the receiver member proximal to the spherical head; and 
 a ring member seated in a groove formed in the receiver member and extending distally beyond the compression cap, the ring member being configured to engage the proximal head to provide sufficient friction between the proximal head and the receiver member to allow the bone screw to be maintained at a desired angular orientation with respect to the receiver member; 
 wherein the compression cap includes one or more detents for receiving a portion of the receiver member therein to retain the compression cap within the receiver member. 
 
     
     
       2. The assembly of  claim 1 , wherein the ring member is configured to expand to fit around at least a portion of the spherical head. 
     
     
       3. A polyaxial bone anchor assembly, comprising:
 a bone screw that includes a shank and a proximal head; 
 a receiver member that defines a rod-receiving portion for receiving a spinal fixation element therein, a distal end of the receiver member including a seat that engages the proximal head to maintain the proximal head disposed therein; 
 a compression cap disposed within the receiver member to lock engage the proximal head; and 
 an engagement feature seated in the receiver member, the engagement feature being configured to engage the proximal head to provide sufficient friction between the proximal head and the receiver member to allow the bone screw to be maintained at a desired angular orientation with respect to the receiver member, 
 wherein the engagement feature includes a separate ring member that circumscribes a portion of the proximal head, and the ring member is disposed within a groove formed in the receiver member, with the proximal head extending proximal of the groove in the receiver member. 
 
     
     
       4. A polyaxial bone anchor assembly, comprising:
 a bone screw that includes a shank and a proximal head; 
 a receiver member that defines a rod-receiving portion for receiving a spinal fixation element therein, a distal end of the receiver member including a seat that engages the proximal head to maintain the proximal head disposed therein; 
 a compression cap disposed within the receiver member to engage the proximal head; and 
 an engagement feature seated in the receiver member, the engagement feature being configured to engage the proximal head to provide sufficient friction between the proximal head and the receiver member to allow the bone screw to be maintained at a desired angular orientation with respect to the receiver member, 
 wherein a proximal surface of the compression cap includes a recess configured to engage the spinal fixation element therewith, 
 wherein at least a portion of the engagement feature bears against a surface of the compression cap, and 
 wherein the engagement feature is movable relative to the compression cap. 
 
     
     
       5. The assembly of  claim 4 , wherein the engagement feature extends distally beyond the compression cap. 
     
     
       6. The assembly of  claim 4 , wherein the rod-receiving portion is defined by opposed sidewalls that are parallel to one another. 
     
     
       7. The assembly of  claim 4 , wherein the engagement feature includes a separate ring member that circumscribes a portion of the proximal head. 
     
     
       8. The assembly of  claim 7 , wherein the ring member has a circular cross-sectional shape or a C-shaped cross-sectional shape. 
     
     
       9. The assembly of  claim 7 , wherein the ring member is disposed within a groove formed in the receiver member. 
     
     
       10. The assembly of  claim 9 , wherein the ring member expands or contracts to be completely disposed within the groove. 
     
     
       11. The assembly of  claim 7 , wherein the ring member is disposed within a groove formed in the proximal head. 
     
     
       12. The assembly of  claim 11 , wherein the ring member expands or contracts to be completely disposed within the groove. 
     
     
       13. The assembly of  claim 4 , wherein the engagement feature includes an expandable portion that is disposed on a distal end of the compression cap. 
     
     
       14. The assembly of  claim 13 , wherein the expandable portion expands around the proximal head to engage the head to provide the sufficient friction that allows the bone screw to be maintained at a desired angular orientation with respect to the receiver member. 
     
     
       15. The assembly of  claim 13 , wherein the expandable portion includes a collet having one or more projections extending therefrom that abut the proximal head. 
     
     
       16. The assembly of  claim 15 , wherein the projections are received in the groove when the proximal head is locked within the receiver member. 
     
     
       17. The assembly of  claim 4 , wherein the compression cap includes one or more pairs of slots that extend from a proximal end of the compression cap towards a distal end of the compression cap. 
     
     
       18. The assembly of  claim 17 , wherein one or more of the pairs of slots are diametrically opposed from one another. 
     
     
       19. The assembly of  claim 4 , wherein the compression cap is spring biased by first and second slots formed on the compression cap. 
     
     
       20. The assembly of  claim 4 , wherein the compression cap includes an axial slot to allow the compression cap to contract or expand with respect to the proximal head to engage the proximal head. 
     
     
       21. The assembly of  claim 4 , wherein the compression cap includes one or more detents for receiving a portion of the receiver member therein to retain the compression cap within the receiver member.